1. Field of the Invention
This invention relates to a new process for strengthening geological formations in underground mines or underground constructions using reaction mixtures based on organic polyisocyanates and polyhydroxyl compounds which also contain special cyclic polysiloxanes as compounds for regulating the foaming process.
2. Description of the Prior Art
The method of strengthening geological formations below ground, in particular in coal mines, by means of polyurethane-forming foams which harden inside the formation is already known (see, for example the Journals Gluckauf (1968), pages 666 to 670; Gluckauff (1977), pages 707 to 711; and Bergbau (1977), pages 124 to 129, and DE-PS Nos. 1,129,894, 1,758,185, 1,784,458, 2,436,029, 2,623,346 and 3,139,395). Mixtures which react to form polyurethanes are also used for sealing off against water and/or gases.
In this process, which is to be regarded as state of the art, the following procedure is generally adopted. The two components of the reaction, the polyisocyanate and the polyalcohol (polyhydroxyl component), are transported separately to a position in front of a bore hole, where they are combined continuously in a static mixer and forced under pressure through the bore hole into the rock formation where the mixture foams up and hardens.
Geological formations which are to be strengthened on this principle, in particular layers of rock below ground, vary considerably in their water content. This means that when reaction mixtures based on the polyisocyanates and the polyether polyols known from polyurethane chemistry are used, the polyisocyanate component reacts in an uncontrolled manner with the water present in the formation, especially in rock layers which have a high water content, to give rise to lightweight polyurea foams which do not always provide sufficient strengthening of the rock formation. Homogeneous and adequate strengthening of the rock formation can only be ensured if controlled and homogeneous foaming takes place to a large extent independently of the water content of the formation. The properties of the resulting foams should be predeterminable by suitable choice of the composition of the reaction mixture, regardless of the water content of the formation. It has been found in the past that the addition of castor oil to the conventional polyether polyols in the polyol component is a first step in this direction. The castor oil, which is generally added in quantities of 5 to 20% by weight, based on the polyol component, reduces the tendency of unwanted side reactions between the polyisocyanate and water so that excessive foaming of the polyurethane resin, in particular in contact with the water present in the formation, and the resulting weakening of the mechanical properties are avoided.
However, castor oil has the disadvantage that it is a naturally occurring product and therefore subject to considerable fluctuations in its properties. Further, since castor oil must be used in the large quantities mentioned above, it constitutes a major cost factor and the price of this natural product is subject to considerable fluctuations and on average is considerably above the cost of conventional polyether polyols.
It was therefore an object of the present invention to provide a new process for strengthening geological formations which is not dependent on the use of castor oil and which ensures the advantages obtained with castor oil.
This problem was solved by the process according to the invention described below, in which certain cyclic polydimethylsiloxanes are used. It was surprisingly found that by adding this auxiliary agent it was possible not only to improve on the advantages obtained with castor oil but also to substantially improve the mechanical properties of the polyurethane plastics. One of the main effects of adding these cyclic polydimethylsiloxanes is to prevent the formation of lightweight, i.e. highly expanded foams even in layers of rock which have a high water content, so that uniformly expanded polyurethane plastics with excellent mechanical properties are obtained by the process according to the invention.
The fact that cyclic polydimethylsiloxanes prevent the formation of highly expanded foams is surprising since it is known from polyurethane chemistry that (linear) polydimethylsiloxanes and polyether polysiloxanes are foam stabilizers and, therefore, have exactly the opposite effect.